1. Field of the Invention
The invention is related to a rotor used in a small motor, particularly a synchronous motor, and relates to the shape of a magnet used in the rotor.
2. Description of the Prior Art
The conventional magnets used in synchronous motor rotors have holes in them for connecting two motor shafts. The shafts are pressed or bonded into the holes.
One method of forming these holes in the magnets is to form them in the pressure-powder molding process by which the magnets are manufactured. Specifically, the mold in which the magnet is formed is shaped to create the necessary holes in the magnets. For isotropic magnets, this molding process can be used to make the holes, because the magnets can be magnetized after the pressure-powder molding process is completed. But for aeolotropic or anisotropic magnets, the hexagonal plate-shaped ferrite crystals in the magnet must be aligned in a certain direction by preliminary magnetization during the pressure-powder molding process. If the magnets are small and if the difference between the outer and inner diameters, i.e. the total wall thickness, is large, the pressure-powder molding process requires a long compression stroke. A long stroke makes it difficult to apply uniform pressure throughout the magnet, so that the ferrite crystals of an aeolotropic magnet cannot be properly aligned. Since aeolotropic magnets yield greater rotational torque than isotropic magnets of the same shape, aeolotropic magnets are preferred for use in motors. Because of the difficulty of forming the holes while maintaining alignment of the ferrite crystals, however, it has not been possible to obtain an aeolotropic magnet with an appropriately strong magnetic field in the radial direction with respect to the axis of the motor shaft.
The alternative method of cutting holes in a magnet with a tool, such as a drill, is usually difficult, except for magnets formed by grinding. Therefore, holes are almost never formed by cutting.
As a result of the above-described difficulties, small magnets having appropriately strong magnetic fields and having outer diameters of approximately 15 mm., inner diameters of approximately 5 mm., and overall lengths of approximately 30 mm., for example, have not been available. Consequently, rotors having either weak magnetic fields or incorporating extremely expensive magnets have been used. It would be desirable to provide rotors incorporating small, strong magnets but without the necessity of molding or cutting holes in the magnets.